1. Field of the Invention
The present invention relates to an STM memory medium and, more particularly, an STM memory medium whose surface smoothness or evenness is remarkably enhanced. It also relates to a method of manufacturing such STM memory medium.
2. Description of the Related Art
STM (or scanning tunnel microscope) has a resolution of atomic size (Binning et al., "Surface Studies By Scanning Tunneling Microscopy". Physical Review Letters, Vol. 49, No. 1, pages 57-61, 1982). An ultra-high density memory which uses this STM as the input/output means has been proposed (U.S. Pat. No. 4,575,822).
FIG. 1 shows the above-proposed STM memory medium 10. Reference numeral 1 in FIG. 1 represents a semiconductor substrate made of silicon, for example. First and second insulating films 2 and 3 are laminated on the semiconductor substrate 1. Silicon oxide and nitride films, for example, are used as these insulating films 2 and 3. Information is written by selectively storing electric charges 4 and 5 at the interface between the insulating films 2 and 3. A probe 6 of the STM is used to write information, and the electric charges 4 and 5 are stored by creating tunnel current at the interface between the insulating films 2 and 3.
Information is written not at areas Data 2 and 4 but at areas Data 1 and 3 in FIG. 1. When tunnel current is monitored by the STM probe 6 under such a recording state as shown in FIG. 1, a characteristic curve shown in FIG. 2A is obtained at each of the Data 2 and 4 areas where no information is written, and a characteristic curve shown in FIG. 2B is obtained at each of the Data 1 and 3 areas where information is written. By detecting whether or not a peak M is present, it is possible to determine whether or not information is written and reading of the information can be achieved.
FIG. 3 shows the STM device used to carry out the writing and reading of information. As shown in FIG. 3, the STM probe 6 is attached to actuators 7, 8 and 9 which are moved in directions x, y and z respectively, and scanning is carried out according to the servo-system by adding voltage to these actuators.
According to this STM memory system, an ultra-high density memory can be realized because the STM has the resolution of atomic size. More specifically, writing and reading of 1 bit can be carried out at an area of 10 nm.times.10 nm, and this enables a memory density of 100 Mbits/cm.sup.2 to be realized.
The above-described STM memory medium 10 is manufactured according to the following method. As shown in FIG. 4, the surface of an Si wafer 11 is heat-oxidized to form an SiO.sub.2 layer 12 and an Si.sub.3 N.sub.4 layer 13 is then formed by chemical vapor deposition ("CVD"). This conventional STM memory medium has the following drawbacks.
The tunnel gap in the STM memory is on the order of a nano-meter and the memory medium used is thus required to have a surface smoothness or evenness of a nano-meter. This is because the scanning of the STM probe 6 at the time when information is written and read is carried out by the servo-system in such a way that voltage is applied to the actuators, to which the probe 6 is attached, so as to keep tunnel current constant, and because the normal distribution of electric charges cannot be read correctly when the surface un-evenness of the memory medium is substantial. This is also because the scanning cannot be carried out at high speed even if information could be written and read.
According to the conventional manufacturing method which has been described above referring to FIG. 4, the top layer 13 of Si.sub.3 N.sub.4 is formed by the CVD. The surface of this Si.sub.3 N.sub.4 layer 13 is thus made relatively rough, as shown in FIG. 4, thereby making it impossible that the Si.sub.3 N.sub.4 layer 13 has such surface smoothness or evenness of nano-meter order as is needed.